1. Field of the Invention
The present invention relates to a method for driving a photo-sensor for improving a response to light. The present method for driving the photo-sensor is applicable to a photo-sensor in an image input unit such as facsimile machine, digital copier or scanner, or a photo-sensor for reading image information at an equi-magnification.
2. Related Background Art
As electronic office equipment such as facsimile machines or digital copiers have become polular, demand for a compact and low cost image input device has increased. Thus, an equi-magnification line sensor which can be directly contacted to a document sheet and does not require a focusing system, or one that allows a short operation distance for the focusing system has become an object of public attention.
The equi-magnification line sensor has the same length as one side of the document sheet and comprises a number of photo-sensors arranged with a high density to attain a high resolution. However, in order to rapidly read information from such a number of photo-sensors, it is important to improve a photo-response time of the photo-sensor.
A photo-sensor having an auxiliary electrode formed on a photo-conductive layer with an insulative layer being interleaved therebetween has been proposed (for example, in Japanese Laid-Open Patent Application No. 239072/1985). By the provision of the auxiliary electrode, an output of the photo-sensor is stabilized and an output proportional to a light intensity is produced.
FIGS. 1 and 2 show structures of photo-sensors having auxiliary electrodes.
In FIG. 1, an auxiliary electrode 2 and an insulative layer 3 are formed on an insulator substrate 1 such as glass or a ceramic, and a semiconductor layer 4 such as CdS.Se or a-Si:H is formed thereon as a photo-conductive layer. A pair of main electrodes 6 and 7 are formed on ohmic contact doping semiconductor layers 5, and a photo-sensing window 8 is formed therebetween.
In the photo-sensor shown in FIG. 2, the elements having the functions similar to those of the photo-sensor of FIG. 1 are designated by like numerals. The substrate 1 is transparent so that light is received through the substrate 1.
FIGS. 3A and 3B show waveforms for explaining a prior art method for driving the photo-sensor.
A high level drive voltage with respect to a potential of the main electrode 6 is applied to the main electrode 7, and a low level voltage V.sub.g =-3V is applied to the auxiliary electrode 2. Under this condition, the number of electrons in the semiconductor layer 4 is small and an output current flowing across the main electrodes is small unless light is detected.
In FIG. 3A, when a light of 100 Luxes is detected, a pulse voltage is applied to the auxiliary electrode 2 and reading is started.
As the pulse voltage rises, the voltage V.sub.g of the auxiliary electrode 2 reaches -4V. Since positive charges corresponding to the capacity of the auxiliary electrode 2 are short, the electrons in the semiconductor layer 4 are swept out of the main electrode 7. As a result, an electron density reduces and the output current rapidly reduces. As the voltage V.sub.g of the auxiliary electrode 2 returns to -3V by the fall of the pulse voltage, the positive charges are in excess and the excess electrons are injected from the main electrode 6 into the semiconductor layer 4 so that the output current rapidly increases, (the change of the output current is shown by a solid line.) As the light pulse falls, the output current gradually decreases. By storing charges in the capacitor with the output current in the read period in a capacitor, the stored charges represent light information of the incident light pulse.
When the voltage V.sub.g of the auxiliary electrode 2 is fixed to -3V instead of applying the pulse voltage, the output current increases as the light pulse rises as shown by a broken line, and it gradually decreases as the light pulse falls.
In FIG. 3B, when a light of 100 Luxes is extinguished and a dark state is created, a pulse voltage is applied to the auxiliary electrode 2 to start reading. A change of output current is shown by a solid line. When the voltage V.sub.g of the auxiliary electrode 2 reaches -4V, the output current rapidly decreases, and when the voltage V.sub.g returns to -3V, it rapidly increases. Again, the charges stored in the capacitor by the output current in the read period represents the light information of the light pulse.
When the pulse voltage is not applied but the voltage V.sub.g of the auxiliary electrode 2 is fixed to -3V, the output current changes in accordance with the rise and fall of the light pulse, as shown by a broken line in FIG. 3B.
In the above drive method, it appears that the rise of the output current is improved by the application of the pulse voltage to the auxiliary electrode 2 as shown in FIG. 3A, but it is not a substantial improvement when the light information is read in accordance with the stored charges.
As shown in FIG. 3B, the previous output current remains even after the application of the pulse voltage, and the output which exactly represents the incident light is not produced.